1. Field of the Invention
The invention relates to an electrolytic gold plating solution. More particularly, the present invention relates to an electroplating solution useful for gold plating of IC packages or the like which are useful in the semiconductor industry.
2. Description of the Related Art
Since gold is very expensive, when gold plating is carried out, it is required from an economical viewpoint to minimize variations in plating thickness of the plated article and to maintain the plate thickness at or near the required level. On the other hand, from a plate quality viewpoint, broad variations in plate thickness cause problems in that wire bondability is deteriorated in areas having a relatively large plate thickness and plate thicknesses are extremely small at concave portions and through holes, resulting in deteriorated heat resistance and solderability. Accordingly, the gold plating procedures for IC packages are required to produce a plate thickness as uniform as possible from the viewpoints of the cost and quality (see, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 56-84495 and 56-108892). However, since gold is deposited in a large amount at the convex or projected portions of the article where the electric current is concentrated due to the fundamental nature of the electroplating, a gold plate having a uniform thickess cannot be obtained. Thus, with the conventional electrolytic gold plating solutions, there have been obtained gold plates only having a throwing power of at most about 50% as calculated from the Field's equation.
Also, where conventional gold plating solutions are used, there can be only obtained either a gold plate having semibright and uneven surfaces and a large crystalline size, or a gold plate having bright and very smooth surfaces and a small crystalline size. This difference in appearance is due to the differences in the additives (so called grain refiners ) added to the gold plating solution for the purpose of improving the quality of the resulting gold plate. Thus, a gold plating solution containing a thallium or lead compound produces the former gold plate having semibright and uneven surfaces and a large crystalline size, while a gold plating solution containing an arsenic compound produces the latter gold plate having bright and very smooth surfaces and a small crystalline size. That is to say, where conventional gold plating solutions are used, only gold plates having one of the above-mentioned appearances can be obtained and gold plates having different appearances cannot be obtained.